Environmental legislation places limits on an industry's ability to dispose of aqueous streams containing nitro-hydroxy-aromatic compounds. Such aqueous streams are produced by a variety of industrial processes, including the nitration of benzene and toluene. The mixed acid technique of nitration results in the formation of undesired contaminants in the nitrated benzene or nitrated toluene product. These contaminants include a variety of nitrated phenols and nitrated cresols, containing one, two or three NO.sub.2 groups. Removal of the nitrated phenols and cresols from the nitrated benzene or toluene product is necessary to prevent interference with subsequent reaction processes performed upon the nitrated benzene and nitrated toluene. By way of example, it is believed that nitrophenolic materials adversely affect the performance of hydrogenation catalysts in the reduction of dinitrotoluene to form toluene-diamine.
As is well known in the art, nitrated phenols and nitrated cresols can be removed from nitrated benzene and nitrated toluene product by contacting the product with an alkaline aqueous solution. The nitrated phenols and nitrated cresols are converted to their water-soluble salts and remain in the aqueous phase after the washing of the nitrated benzene or nitrated toluene product, thereby freeing the product of the contaminants. It is the disposal of the aqueous phase containing the water-soluble salts of nitrated phenols and nitrated cresols which is limited by environmental legislation.
The prior art has addressed this disposal problem in several ways. Initially, it should be noted that biological treatment of aqueous waste streams containing the salts of nitro-hydroxy-aromatic compounds is considered impossible where such compounds are present in excess of as little as 20 ppm. Nitro-hydroxy-aromatic compounds poison the bacteria utilized in biological treatment. Accordingly, if biological treatment of waste streams is to occur, the nitro-hydroxy-aromatic compounds must be largely removed from the waste stream. Additionally, incineration of the aqueous waste stream is considered to be too energy-intensive due to the relatively low levels, for incineration purposes, of the nitro-hydroxy-aromatic compounds (generally less than two percent by weight).
U.S. Pat. No. 4,113,615 discloses a method for removing organic contaminants from wastewater through adsorption onto char. This method requires the addition of char to the aqueous stream, adjustment of the pH of the stream to acidic or near acidic conditions, and filtration or separation of the char from the aqueous stream once adsorption has occurred. The method results in the need for disposal of the contaminated char, which the patent suggests may occur by landfill.
U.S. Pat. No. 4,230,567 discloses a process for destruction of nitro-hydroxy aromatic compounds in aqueous waste streams by heating the waste stream to temperatures preferably between 200.degree. and 350.degree. C., under pressure, and in the absence of oxygen. Destruction of the nitro-hydroxy-aromatic compounds apparently occurs after maintenance of the elevated temperature for a period of 15 to 30 minutes. It is claimed that the resultant waste stream can then be biologically treated without adverse impact upon the treating bacteria.
U.S. Pat. No. 4,597,875 discloses a process for removal of nitrated phenols and nitrated cresols from the alkaline wastewater stream generated in the nitration of aromatic compounds, by precipitating the nitrophenols and nitrocresols from the wastewater stream. Precipitation is achieved by adding acid to the alkaline wastewater stream until the water-soluble salts of the contaminants are converted to water-insoluble organics. The precipitated organic phase can then be disposed of through incineration. However, the percentage removal of contaminants achieved by this method appears inadequate to allow biologic treatment of the resulting aqueous phase.
U.S. Pat. No. 4,604,214 discloses a method for removing nitrocresols from dinitrotoluene waste streams using Fentons reagent. This method requires the acidification of the waste stream to a pH of between 3 and 4, with the subsequent addition of hydrogen peroxide and ferrous ion resulting in the oxidation of the organic contaminants. Removal of the ferrous salt from the waste stream is then required.